Film-Tech Forum ARCHIVE: Hot Christie platter

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Posted by Michael Schaffer (Member # 1204) on 01-27-2006, 03:02 AM:

Today I got a call saying that the projectionist got shocked by an AW3R. Our electrician happened to be in the area, so we took a look at the platter together and found that there was indeed 120VAC on exposed (unpainted) surfaces. It turned out that the problem appears to have been caused by the dual (ungrounded) convenience outlet of the SLC console the platter was plugged in. When it was plugged into the other outlet of the pair, the voltage to ground was only 30VAC. When plugged into a (grounded) wall outlet, the voltage disappeared. So far, so good. What I haven't been able to figure out yet is how the apparently damaged convenience outlet could cause the voltage to appear on the platter chassis. If there was a short somewhere inside the platter (and from inspecting the wiring, motors, and switches, it didn't appear to be, also all decks were fully operational), it should have tripped the breaker, at least when it was plugged into the grounded outlet. But it didn't. What did we miss?

Posted by Monte L Fullmer (Member # 2797) on 01-27-2006, 05:40 AM:

Did the sockets show any open neutrals with a tester? Most electrical wiring is to attach hot and neutral on one end of the dbl socket and there could have been a break in the brass connecting the other socket, and an open neutral could have been the culprit.

just a guess even though I've seen it happen before...

Posted by Louis Bornwasser (Member # 3063) on 01-27-2006, 11:23 AM:

Very important that AW-3 be grounded; not for the safety reasons,, but for static reasons.

Also, keep in mind that 120 v hot is connected directly to the motor on this platter. The speed control is in the return-to-neutral wire. Louis

Posted by Michael Schaffer (Member # 1204) on 01-27-2006, 12:58 PM:

But how would the motor wiring come into play here? We actually unplugged all the motors at one point, the voltage was still there.

quote: Monte L Fullmer
Did the sockets show any open neutrals with a tester? Most electrical wiring is to attach hot and neutral on one end of the dbl socket and there could have been a break in the brass connecting the other socket, and an open neutral could have been the culprit.

But I guess then the platter wouldn't have run, would it? It did, and at normal speeds.

Posted by Monte L Fullmer (Member # 2797) on 01-28-2006, 04:00 AM:

True, and just as I mentioned..a guess..not a factal statement in helping you out. Just that you had a hot heading towards the platter and was hunting for a neutral to complete the circuit and was using one in an odd way to where the operator got bit.

I seem to want to point to a defective console socket since the platter unit ran fine on the wall socket without any voltage leaks though the VOM readings and you've "condemned" that section of the test with platter plugged into the wall socket.

Louis - isn't that kinda odd that a control would be on the neutral side instead of the hot side? Course, I'm not too all familiar with the wiring of the AW3, but isn't it common knowledge that a controlled wire should be the hot one instead of the neutral one? Maybe, you can please enlighten me on this for future reference.

thx-Monte

Posted by Mark Gulbrandsen (Member # 72) on 01-28-2006, 05:48 AM:

Have had at least two AW-3's in past years with shorts in the motor plugs. Its also a common problem in the MUT socket.

Mark

Posted by John Pytlak (Member # 331) on 01-30-2006, 01:35 PM:

With an ungrounded frame, any electrical leakage can make the metal frame "hot". Could be as simple as moisture in motor windings, or carbon dust from brushes, a frayed wire, or a sliver of solder that isn't where it is supposed to be.

Any industrial equipment that is not "double insulated" must have the frame connected to ground.

Posted by Louis Bornwasser (Member # 3063) on 01-30-2006, 01:53 PM:

Monte: I also consider it odd that the neutral is the side the control is on.

John: Many times on AW-3 motors, the carbon dust builds up on the bottom of the motor and, when shipping out, the motor acts as a generator, causing an arc over from neutral to ground. This failure mode show up as "motor running full speed." Louis

Posted by Michael Schaffer (Member # 1204) on 01-30-2006, 01:58 PM:

It didn't occur to me that there might be a small leak somewhere, but with a resistance high enough so that the currents going to ground from there (when plugged into a grounded outlet) would trip the breaker. I talked to Bevan and he pointed that out to me. Good thing the older versions of that platter have the plate where the AC chord is coming in riveted to the tree! Drilling is so much fun.
I unplugged all the motors, but the voltage is still there. There were also no problems with the wiring in the control card compartment. I think it must be in the bottom of the platter where the AC chord comes in.
Still, what I don't get is, why is the voltage between the ungrounded frame and ground 120VAC when plugged into one of the pair of (ungrounded) convenience outlets on the SLC console but only 30VAC when plugged into the other of the same pair? Both outlets share one set of hot and neutral wiring.

Posted by John Hegel (Member # 628) on 01-31-2006, 12:09 AM:

I'll take a stab at this one.

It sounds like the fault could be in the cord itself. The difference in outlets might be related to how the cord/short is sitting and making contact.

If you were losing your neutral; voltage could bleed through the transformer, assuming there is one and the neutral and ground are bonded at the frame.